Amorphous copolyesters are generally defined as copolyesters that do not show a substantial melting point by differential scanning calorimetry when scanned at a rate of 20.degree. C./min. These copolyesters are typically based on terephthalic acid, isophthalic acid, ethylene glycol, neopentyl glycol and 1,4-cyclohexanedimethanol. It is generally known in the art that amorphous copolyesters possess a combination of desirable properties, such as excellent clarity and color, toughness, chemical resistance and ease of processing. Accordingly, such copolyesters are known to be useful for the manufacture of extruded sheets, packaging materials, and parts for medical devices. However, when compared to other families of materials such as polycarbonates based on bisphenol A and certain acrylic resins like polymethyl methacrylate, amorphous copolyesters generally have lower glass transition temperatures (Tg) and lower heat distortion temperatures (HDT). The lower Tg and HDT of amorphous polyesters inhibit their application where resistance to thermal deformation is important, for example, green house panels, skylights and other products prepared by extrusion or injection molding processes.
It is also important that amorphous copolyesters have a low shear melt viscosity. Unlike semi-crystalline polyesters, where ultimate molecular weight values can be raised by polymerization in the crystalline or solid state, high molecular weight amorphous copolyesters must be obtained directly in the melt phase polymerization. It is not possible or it is too expensive to render the amorphous copolyesters into a crystalline pellet or particle form. Further, when heated to the temperatures required for solid state polymerization, amorphous pellets or particles flow together and stick together.
Accordingly, there is a need for amorphous copolyesters having sufficiently low shear melt viscosities allowing the copolyesters to flow through the reactor at a practical rate allowing the copolyesters to build to a useful molecular weight. It is known in the art that the molecular weight of copolyesters can be conveniently measured in terms of its inherent viscosity ("IV"). Generally, an IV of 0.65 or higher is necessary for useful mechanical properties. Further, the maximum melt viscosity at 1 radian/second (shear rate) generally ranges from about 10,000 poise to about 12,000 poise at the temperature of the melt phase polymerization which is commonly in the range of about 260 to about 290.degree. C.
Crystalline copolyesters with good heat resistance and high strength are disclosed by Japanese Patent No. 08295731. These copolyesters are based on terephthalic acid as the acid component and a mixture of 1,4-cyclohexanedimethanol, ethylene glycol, and ethylene oxide adducts of bisphenol as the glycol component. Amorphous copolyesters, however, are not disclosed.
A process for preparing polyesters with good transparency and useful in the production of bottles and films is disclosed by Japanese Patent No. 08283398. The process comprises the use of a specific antimony compound as a polycondensation catalyst. Polyester compositions for use in the preparation of optical instruments are disclosed by Japanese Patent No. 4173837. These copolyester compositions comprise an acid component consisting of terephthalic acid and a diol component consisting of 4,4'-bis(.beta.-hydroxyethoxy)-diphenylsulfone (2,2'-(sulfonylbis-(4,1-phenyleneoxy))bis(ethanol)) and at least one aliphatic or alicyclic diol having from 2 to 12 carbon atoms and containing a specific combination of at least one metal and phosphorous. According to this Japanese patent, the specific combination of metal and phosphorous is necessary to obtain polyesters with sufficient transparency, hue, Tg, and heat resistance.
Japanese Patent No. 50096692 relates to a process for preparing polyesters from (A) a bifunctional carboxylic acid component composed primarily of terephthalic acid and/or an ester forming derivative thereof, (B) a diol with an aromatic nuclei composed primarily of 4,4'-bis-(.alpha.-hydroxyalkoxy)-diphenylsulfone (2,2'-(sulfonylbis-(4,1-phenyleneoxy))bis(ethanol)) and another diol. According to this Japanese patent, specific property limitations occur when a third diol, such as neopentyl glycol is incorporated.
Polyester sheets composed of ethylene glycol based glycols and terephthalic acid based dicarboxylic acids and containing diphenylsulfone and cyclohexane groups are disclosed by Japanese Patent No. 5279464. However, in order to obtain polyesters with superior transparency and heat resistant properties, the polyesters must contain both diphenylsulfone and cyclohexane groups at low levels.
U.S. Pat. No. 5,183,863 discloses viscoelastic resin compositions for vibration damping material. The resin comprises (A) at least one amorphous polyester resin of low specific gravity in which more than 40 mole % of the dibasic acid moiety is of aromatic type, (B) at least one amorphous polyester resin of high specific gravity in which more than 80 mole % of the dibasic acid is of aromatic type, and at least one hardener. As shown by Table 1 of the patent, the polyester resins according to this patent have relatively low Tg values ranging from -33 to 68.degree. C.
U.S. Pat. Nos. 5,356,989 and 5,510,417 disclose aqueous dispersions suitable for use as coatings, paints, or adhesives which comprises a polyester resin, a water soluble organic compound, water, and a neutralizer. The polyester resins contain a polycarboxylic acid component and a polyol component. The glass transition temperature of the polyester resins according to these patents, however, range from -30.degree. C. to 80.degree. C.
Copolyesters randomly composed of a first repeating unit containing a naphthalene ring structure and a second containing a naphthalene ring and an additional aryl ether linkage incorporated into the main chain are disclosed by U.S. Pat. No. 5,663,238. According to this patent, the copolyesters have improved solubility and are useful in various applications such as paints, varnishes, and structural adhesives. However, these properties can only be obtained with the specific combination of naphthalene and additional aryl ether linkage.
Accordingly, there remains a need for high IV and low melt viscosity amorphous copolyesters having sufficiently high glass transition temperatures and enhanced heat distortion temperatures yielding copolyesters with enhanced mechanical properties, i.e., toughness, without requiring the specific parameters described above. The invention answers this need.